Measurement and calibration of AC signals is much more difficult than in the case of DC. One technique is to apply the AC signal to a heater element adjacent to or contacting a temperature sensor such as one junction of a bimetallic junction thermocouple. According to the Seebeck effect, heating such a junction will induce a current in the wires. (In practice, a very high impedance device, such as a voltmeter, typically is interposed in the circuit of the two wires so that current flow is negligible). The joule heating in the heater, caused by the AC input signal, induces a voltage in the thermocouple. This thermocouple voltage is proportional to the temperature difference between the bimetallic junction adjacent to the heater element and a second, cold or reference temperature junction.
In one type of known method for measurement of AC voltages, the unknown AC signal and an easily measurable DC signal are separately applied to a heater associated with a electrothermal sensor, such as a thermocouple. First, when the AC signal is connected to the heater, the thermocouple voltage is nulled to a stable voltage source by adjustment of a potentiometer, which is adjusted to apply an equal and opposite voltage to that generated by the thermocouple. Then, without changing the potentiometer setting, an easily measured DC voltage is substituted for the AC signal applied to the heater element, which again induces a voltage in the thermocouple. The DC voltage amplitude to the heater is increased, increasing the heater temperature, until the voltage generated by the thermocouple is again equal and opposite to the voltage previously set by the potentiometer during the AC measuring cycle. When this null condition is reached, the amplitude of the DC voltage is measured. This DC amplitude is thereby equal to the RMS amplitude value of the AC signal.
Various methods and apparatus for AC signal measurement or calibration to a thermocouple are disclosed, for example, in U.S. Pat. Nos. 3,723,845, 4,659,910 and 4,695,793.
In another method and apparatus, disclosed in U.S. Pat. No. 4,901,006, the same thermal sensing means are used to determine the temperature produced in a heater as a result of two signals. The thermal sensing means and heater are maintained in a fairly narrow temperature range during the measurement process. In embodiments described there, this is accomplished by use of two heaters in thermal contact each connected to a separate signal and by having the thermal sensor sense the net temperature produced by both heaters, producing a difference signal.